fn back_up_legacy_claude_md(wd: &Workdir, content: &str) -> Result<()>Expand description
Copy a legacy CLAUDE.md to CLAUDE_MD_PRE_ADOPT_BACKUP before it is
adopted. Whole or not at all (a failed write removes its partial copy);
a copy left by an earlier attempt counts only if it is this file, whole
(Codex on #4131).